(a) Field of the Invention:
The present invention relates to a lens used for the recording and playing back of optical disks, etc., especially a graded refractive index (GRIN) lens.
(b) Description of the Prior Art:
In an objective lens system used for the recording and playing back of information which is recorded with high density on recording medium such as optical disks, etc., it is necessary to have resolving power of about 1 .mu.m.
An objective lens system of the above mentioned kind employs an arrangement wherein a lens system itself is directly driven for the purpose of autofocusing and tracking and, therefore, it is necessary for the lens system to be as compact and light as possible.
In order to improve the resolving power of a lens system, which is one of the necessities as mentioned above, it is enough to sufficiently correct only axial aberration because the optical disk makes use of the axial image point of an optical system. In view of the directional error of the incident ray flux or the influence of decentering by errors generated in the setting up of the lens, it is desirable to sufficiently correct aberration in the range of diameters of no less than about 0.2 mm from the optical axis as a center on the image surface.
If the lens system satisfying the above mentioned condition comprises spherical homogeneous lenses, the lens system will have to be a combination of a plurality of lenses and the length of the lens system will accordingly be long.
In recent years, a plastic lens both surfaces of which are formed aspherically has been developed and there has been seen remarkable progress in the related technology from the viewpoint of making the lens system compact and light.
On the other hand, for economy of manufacture and for reduction of size and weight, the lens systems in which a GRIN lens is employed have been reported. In these GRIN lens systems, at least one refracting surface is formed spherically so that aberrations are corrected.
The reported GRIN lens systems include the lens system disclosed in Japanese Published Unexamined patent application No. 6354/80 in which only spherical aberration is corrected. In the lens system disclosed in Japanese Published Unexamined patent application No. 62815/84, consideration is also given to the correction of off-axial coma aberration and the system is arranged to keep the necessary working distance. However, because the shape of the lens is strongly meniscus, the designed working distance cannot be secured in practice. In order to secure the practical working distance, the desired lens shape should be a biconvex lens. The lens system disclosed in Japanese Published Unexamined patent application No. 122512/83 involves a system comprising a biconvex lens, in which the correction of aberration, however, cannot be said to be on the level of practical use. In the lens system disclosed in the latter two prior art references, as for the correction of off-axial aberrations, only that of coma (sine condition) is taken into account. In order to obtain the lens system having wide visual field, etc., it is also necessary to sufficiently correct astigmatism.